The present invention relates to the field of imaging or position sensitive detectors and, in particular, to two-dimensional detectors used with microchannel plates.
Microchannel plates are often used for image amplification or intensification. FIGS. 1A and 1B are plane and cross-sectional views, respectively, of one such microchannel plate 10. Microchannel plate 10 includes a structure 15, which is typically leaded glass. Structure 15 has a metallic coating on both surfaces 17 and 18, and a high voltage is placed across the two surfaces when the plate is in operation. Piercing glass 15 is a two-dimensional array of a plurality of channels, typically 10,000 to 10,000,000, arranged in some type of regular matrix. For simplicity, FIGS. 1A and 1B show much fewer channels.
Each channel in FIGS. 1A and 1B is an electron multiplier which receives electrons at its inputs, but the inputs to the plates could also be protons, ions, photons or other similar particles or events. The diameters of the microchannels are on the order of tens of microns, and the channels themselves are usually biased at some slight angle with respect to surfaces 17 and 18. The angle in FIG. 1B is approximately 8.degree. .
Of course, FIGS. 1A and 1B show only one type of microchannel plate. There are many other configurations, for example a chevron configuration. The following description is not limited to the particular configuration shown in FIGS. 1A and 1B.
The operation of microchannel plates is well known. Very simply, electrons enter one end of a channel, strike the sides of that channel and dislodge other electrons. The dislodged electrons in turn dislodge additional electrons so that eventually many electrons leave the channel in response to each electron that enters. A typical multiplication factor for microchannel plates is between 10,000 and 10,000,000. Additional details regarding the construction and operation of microchannel plates can be found in J. Wiza, "Microchannel Plate Detectors," Nuclear Instruments and Methods 587, Vol. 162 (1979), which is incorporated herein by reference.
As previously indicated, microchannel plates are often used in image intensification devices to amplify image data either for display, for example on a phosphor screen, or for later data processing. Image intensification has important military uses, such as for night vision devices and for optical sensing in spacecraft. Image intensification devices for military uses must be able to locate objects with great speed and accuracy. For spacecraft applications, such devices need great accuracy with relatively simple circuit configuration to reduce power consumption. The speed and accuracy of such an image intensification device depends not only on the microchannel plate configuration, but also on certain characteristics of the detector, which captures and measures the output of microchannel plates, and the electronics coupled to the detector.
The anode detectors, or anodes for short, measure the number of electrons which leave the microchannel plate detector and strike the anodes. The electrons incident on a metal anode create an electric potential, usually a pulse, whose change in voltage, V, corresponds to the number of electrons incident on an anode. The voltage change may be determined by: EQU V=Q/C.sub.anode
where Q=1.6.times.10.sup.-19 (Coulomb/electron) x number of electrons, and C.sub.anode is the capacitance of the anode.
Several different types of anode detectors have been used in the past in both military and space applications. Generally, all the anode detectors are used to discriminate incoming energy, for example, optical or ionizing radiation.
The SSANACON method, which stands for Self Scanned Anode Array Image Converter is described in A. Broadfoot and B. Sandel, "Self-Scanned Anode Array with a Microchannel Plate Electron Multiplier: the SSANACON," Applied Optics pp. 1533-38, Vol. 16, No. 6 (June 1977), which is incorporated herein by reference. The device described in that article contains an anode array with one hundred and twenty-eight (128) 3 mm long rectangular anodes aligned in parallel on 100 um centers. The anodes were mounted to receive the outputs from microchannels at 20 um centers. The anodes are alternately connected by FET shift register control switches to one of two video lines denoted even and odd. Photoevent counting with this device uses a fast interrogation of the anode array: a clock frequency of 200 kHz or an anode sampling rate of 3 kHz.
One problem with the SSANACON is that it can discriminate in one direction only, that direction being along the axis of the 128-anode array. Furthermore, the shift register data collection arrangement coupled to the anodes lacks flexibility, thus limiting the modes of accessing or processing the anodes.
A different method of detection using microchannel plates, which does allow two-dimensional discrimination, is discussed in C. Martin, P. Jelinsky, M. Lampton, R. Malina, and H. Anger, "Wedge-and-Strip Anodes for Centroid-Finding Position Sensitive Photon and Particle Detectors," American Institute of Physics, Rev. Sci. Instrum., pp. 1067-1074, Vol. 52, No. 7 (July 1981), which is also incorporated herein by reference. The wedge-and-strip method described in this publication uses an array of alternating wedge shaped and strip anodes. The width of the wedge-shaped anodes varies linearly in a direction perpendicular to the axis of the anode array (i.e., is triangular). The strips are rectangular and have different widths. All the anodes are coupled to signal processing electronics which computes values for the location of the centroid of the incident energy in the X and Y directions. The strips are used to determine centroid position along the Y or array axis, and the wedges are used to detect the centroid position in the X axis, which is the axis perpendicular to the array axis.
One disadvantage of the wedge and-strip anodes is that the processing electronics connected to the anodes is relatively complex and becomes more so as the number of anodes increases. The electronics also provides no flexibility for processing. In addition, since the anodes are relatively large, they have a high capacitance and therefore a poor signal-to noise ratio.
A slightly different version of the wedge-and-strip anode is shown in Praq, U.S. Pat. No. 3,934,143, which shows a number of alternatinq right-triangular electrodes in an ionizing radiation detector. The electrodes extend along the upper surface of a plate which has a common electrode covering substantially the entire surface of the other side of the plate. The triangular electrodes are of p-conductive silicon and are separated by small slices of silicon dioxide. Each electrode is coupled to several special purpose adders, multipliers, and other special purpose circuitry to calculate the position in the X and Y axis of the centroid of the incident energy. The disadvantages of this device, however, are generally the same as with the wedge and strip device and method, except the anodes in Praq do not appear to be as large as those in wedge-and strip device.
One object of this invention is an accurate and fast two-dimensional position detector for use with a microchannel plate.
Another object of this invention is a microchannel plate position detector which is flexible enough to accommodate different numbers and orientations of anodes, as well as different sequences for scanning the anodes, without requiring substantial added circuitry, cost, or complexity.
Additional objects and advantages of this invention are set forth in part in the description which follows and in part are obvious from that description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by the methods and apparatus particularly pointed out in the appended claims.